(i) Field of the Invention
The present invention relates to a carbon material for electrodes which are used in electrochemical detectors, sensors for environmental analysis, sensors for pathologic inspection, and probe electrodes for detection which are required to be harmless and nonpoisonous to biological systems and foods, and it also relates to a process for preparing the carbon material.
(ii) Description of the Prior Art
In a wide field of electrochemical measuring techniques, an abbreviated rapid analysis which has been swiftly developed in recent years and which utilizes a sensor as a detecting means permits performing a measurement with very high selectivity and high sensitivity. Therefore, the use of the abbreviated rapid analysis is begun to analyze and evaluate biological samples and environmental samples containing trace amounts of many compounds which are components to be measured.
Furthermore, nowadays, it has been very important to obtain, for example, biological information of such specific substances on a cellular level in vivo in situ in local parts of the biological system and the like by the use of a sensor electrode capable of detecting them with high sensitivity. For this purpose, the electrode is used as follows: The electrode is disposed or stuck in the vicinity of an aimed cell of an organism to give physical, chemical and electrical stimulation to the cell, so that a response substance is released or a chemical change occurs, with the result that a specific substance is produced. At this time, the electrode is required to select and measure the thus produced specific substance.
A selected electrode material for a working electrode of the electrochemical detector has a large influence on a kind of substance to be measured and the possibility of quantitative analysis. Heretofore, as such a working electrode for voltammetry, there have been investigated a mercury dropping electrode, a static mercury electrode, platinum, gold, gold amalgam, copper and various conformations of carbon (glass like carbon, carbon fiber, carbon paste and pyrolitic graphite), but the mercury electrode and the glass like carbon are predominantly used as an electrode for polarographies and as an electrode for electrochemical detectors, respectively.
However, the polarography has a problem of safety, since it uses toxic mercury. The glass like carbon also has many drawbacks which should be eliminated. That is, the glass like carbon has low current sensitivity, and when a high positive potential is applied thereto through an electrolyte, the reproducibility on the surface of the electrode deteriorates. Thus, a utilizable potential range is limited to about +1.0 V or less. In addition, the glass like carbon is largely affected by a pretreatment such as an oxidizing treatment, and so the reproducibility of the measurement is unreliable.
The platinum electrode cannot be used on the negative side of a hydrogen generating potential, and additionally its pretreatment is also difficult.
The gold electrode is less resistant to halide ions. Since these electrodes are made of the metals, they are dissolved when used, thereby releasing toxic ions which harm the biological system. However, there are not good electrodes which can take the place of them, and hence these electrodes have been still unavoidably used. Moreover, in these days, a carbon micro-electrode comprising one carbon fiber (having a diameter of several micrometers) has been begun to be on the market, but according to a pursuit test by the present inventors, it has been apparent that the carbon micro-electrode has the large variation of electrochemical properties, poor reproducibility of data, and very low reliability.
Graphite has a wide polarized potential domain, an electrode reaction activity and a less toxicity to the organisms, and therefore it is a useful material. However, the graphite is poor in mechanical strength, and when it is used singly, an electrolyte penetrates into its structure. For the prevention of this penetration, the graphite is required to be impregnated with an oil or resin. Nevertheless, in the system in which even a small amount of an organic solvent is contained, the material with which the graphite has been impregnated is dissolved therein, and the variation of the electrochemical properties increases, so that the reproducibility of data is poor.